In recent years, video cameras utilizing solid state imaging devices have entering into practical use. However, the solid state imaging device is extremely expensive, since the production yield of the solid state imaging device, is low. This is because the degree of integration is high, chip area are large and signals to be handled is analog signals, are low. This therefore, is a great obstacle against popularization of video cameras using the solid state imaging device.
Also, when there are picture defects such as white dots or black dots, it is necessary to correct the defects in a signal processing circuit.
Hereafter, the defect correction apparatus of the prior art is elucidated. This example is reported in the Technical Report of the Institute of Television Engineers of Japan (Television Gakkai Gijutsu Houkoku Vol. 7, No. 14, pages 19-24).
FIG. 1 shows a block diagram of the defect correction apparatus of this prior art, which includes solid state imaging device 1, pre-amplifier 2, defect correction circuit 3, driving circuit 4 and memory 5.
The operation of this conventional imaging apparatus is elucidated in the following. A signal from the imaging device 1 is amplified by the pre-amplifier 2 to a predetermined level. Those parts of the amplified signal from pixels that contain defects produced by failures of the imaging device are converted by a defect correction circuit 3 into a signal which does not include a defect. This converted signal is used as an input to the next processing circuit. Judgement of whether the input signal includes the defect or not is carried out by using data stored in a memory 5 (recording circuit) which shows the position of defects.
Details of the signal processing relating to the defect correction are elucidated referring to FIG. 2. The imaging signal, after the defect correction, is converted into necessary principal color signals R (red color signal), G (green color signal) and B (blue color signal), and is output after conversion, for instance, as an NTSC signal. A method shown in FIG. 2 is used. As the defect correction method Numeral 101 is a clamp circuit, 102 is a delay circuit and 103 is an averaging circuit. The method of FIG. 2(a) performs defect correction using making a substitution by a signal which is prior to the signal in which the defects are arising. FIGS. 3(a) and (b) show methods of switching-over.
A signal of n-th order shown by hatching in FIG. 3(a) is a signal containing a failure such as the described defect. The n-th position is preliminarily stored in the memory 5 as the failure signal, and in compliance with scanning of the imaging device, the failure correction signal is sequentially output. When the failure correction signal is output, it is switched for the n-2-th signal which has no failure such as the defect, by the switching 104. This yields a signal as shown in FIG. 3. FIG. 2(b) shows a similar correction apparatus. This uses a method, such that when a failure correction is necessary, an averaging processing is made by using signals before and after the picture element. FIG. 3(c) shows this situation. A signal of 1/2{(n-2)+(n+2)}-th signal used. The signal shown by .DELTA. is the signal substituted for the signal of nth order (that contains the errors).
However, the above-mentioned conventional constitution has a problem that errors are produced corresponding to kinds of pictures picked up or changes in the pictures, thereby to lower the picture quality, since signal after making correction of a failure such as defect is corrected by using signals of positions of the same relations for all picked up pictures.